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Linear Photodiode Feedback Automatic Alignment System

Publishing Venue

IBM

Related People

Khoury, HA: AUTHOR

Abstract

Shown is an automatic wafer alignment system, which is based on the detection of the X-Y position of an alignment mask in the kerf of large-scale integrated circuit chips on a wafer or on special alignment sites. The image of the alignment mark, consisting of a cross, is projected optically onto the surface of a linear array of photodiodes.

Country

United States

Language

English (United States)

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Linear Photodiode Feedback Automatic Alignment System

Shown is an automatic wafer alignment system, which is based on the
detection of the X-Y position of an alignment mask in the kerf of large-scale
integrated circuit chips on a wafer or on special alignment sites. The image of the
alignment mark, consisting of a cross, is projected optically onto the surface of a
linear array of photodiodes.

The position error of the image relative to the linear array is determined by
scanning the array, and determining the location of each leg of the cross within
the array. Location of the image is determined by contrast differences between
the alignment mark and its background, which cause corresponding voltage
differences at the output of the diodes. Data acquisition and reduction is
performed via a computer.

Once the position of the image is known, corresponding X-Y error signals are
generated which are fed to an X-Y table in the closed-loop servo system. The
servo system may be operated in either digital and/or analog modes.

As shown in Fig. 1, an illumination system 1 directs a beam of light through
aperture 2. By beam splitter 3 and objective lens 4, the image of the aperture is
reduced and focused on wafer 5. A filter 6 may be utilized to block light
frequencies which may expose the photoresist. The reflected light from the
alignment mark is collected by the objective lens 4 and focused, via beam splitter
7 and relay lens 8, on the surface of the feedback detector system 9. Thus, the
projected image of the alignment mark is optically relayed to the surface of the
feedback detector 9.

As shown in Fig. 2, a linear array of 256 photodiodes 10 is provided
consisting of 0.001 inch square diode elements on a center-to-center spacing of 0.001 inch. Initially, the diode array is vertic...